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To use lower temperatures (which are more efficient) you need a higher flow rate. Narrower than 15mm pipes won't provide enough flow, and even 15mm pipes won't be wide enough in some systems. Although the perception that ASHP means you need big wide pipes isn't necessarily right.
55° is the maximum flow temp in the new building regs (part L) but to get maximum efficiency you want 45°.
We're putting underfloor everywhere and installing lots of insulation so we can just swap in an ASHP in future (I think they're probably too noisy for us right now). As part of that I reckon we'll have at least 22mm pipes to the manifolds, although I haven't confirmed that detail yet.
Fox-
gotcha
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I have been doing tours of houses with ASHP to help decide on which one’s best for us. Noise is a big concern.
I’ve looked at several Mitsubishi Ecodan, Daikin, Samsung and Vaillant units. The units with by far the quietest and least offensive sonic profile (frequency & harmonics) are the 5/7kW Vaillant models.
Everything else is noticeably louder and harsher-sounding, especially the two Ecodan units I heard.
We’re getting a 7kW Vaillant, which doesn’t sound too dissimilar to a boiler exhaust flue.
Sumo
ectoplasmosis
I don't understand how they're less efficient. More heat loss? Longer to heat up the radiators?